This invention relates to methods and apparatus for generating controlled layer thickness gradients in multilayered polymeric bodies which are useful as optically reflective articles.
The coextrusion of multilayer sheets and other articles wherein individual layer thicknesses are on the order of microns is known in the art. For example, Schrenk et al, U.S. Pat. Nos. 3,773,882 and 3,884,606, teach devices which prepare multilayered coextruded thermoplastic polymeric materials having substantially uniform layer thicknesses. The feedblock of the coextrusion device receives streams of diverse thermoplastic polymeric materials from sources such as heat plastifying extruders. The streams of these thermoplastic materials are passed to a mechanical manipulating section within the feedblock. This section serves to rearrange the original streams into a single multilayered stream having the number of layers desired in the final body.
Optionally, this multilayered stream may be subsequently passed through a series of layer multiplying means (sometimes termed interfacial surface generators) in order to increase further the number of layers in the final body as is described in Schrenk et al, U.S. Pat. Nos. 3,759,647, 5,094,793, and 5,094,788. The multilayered stream is then passed into an extrusion die which is so constructed and arranged that streamlined flow is maintained therein. Such an extrusion die is described in Chisholm et al, U.S. Pat. No. 3,557,265. The resultant product is extruded through the die to form a multilayered body in which each layer is generally parallel to the major surface of adjacent layers. This technology has been called microlayer coextrusion technology because of the thinness of the layers which are formed. Microlayer coextrusion is to be distinguished from conventional multilayer coextrusion which typically involves the production of less than about fifteen layers, each having thicknesses which may be from one to two orders of magnitude greater than the layer thicknesses produced in microlayer coextrusion.
Such multilayer polymeric bodies have found use as optically reflective sheets and films. For example, Alfrey, Jr. et al, U.S. Pat. No. 3,711,176, and Radford et al, "Reflectivity of Iridescent Coextruded Multilayered Plastic Films", Polymer and Engineering Science, Vol. 13, No. 3, pp. 216-221 (May 1973), teach a multilayered highly reflective thermoplastic body fabricated using the multilayer coextrusion devices of Schrenk, discussed above. The reflective optically thin film layers of Alfrey, Jr. et al and Radford et al relied on the constructive interference of light to produce reflected visible, ultraviolet, and/or infrared portions of the electromagnetic spectrum. Further, as such optically thin films are highly reflective at wavelengths where there is constructive interference, the multilayer films were made up of only a few hundred layers or less.
Other multilayer polymeric bodies also use constructive interference of light to produce useful reflective articles. For example, commonly-assigned Schrenk et al, U.S. patent application Ser. No. 07/969,710, filed Oct. 29, 1992, and entitled "Formable Reflective Multilayer Body", teach broad bandwidth reflective films which utilize layer thickness gradients. Commonly-assigned Wheatley et al, U.S. patent application Ser. No. 07/963,305, filed Oct. 16, 1992, and entitled "All-Polymeric Cold Mirror" teach a multilayer body which reflects visible wavelengths while transmitting a substantial portion of infrared wavelengths.
For such multilayer films which use constructive interference, the location of the first order reflectance peak for a typical two component film is defined by the equation: EQU .gamma..sub.I =2(N.sub.1 d.sub.1 +N.sub.2 d.sub.2) (Equation 1)
where, .gamma..sub.I is the wavelength of first order reflection in nanometers, N.sub.1 and N.sub.2 are the refractive indices of the first and second polymers, and d.sub.1 and d.sub.2 are the respective layer thicknesses of those polymers in nanometers. As can be seen, a controlled distribution of the layer thicknesses, d, will spread the first order reflectance peak over a desired wavelength range. The shorter reflected wavelengths correspond to thinner layers in the body while longer reflected wavelengths correspond to thicker layers. If the layer thicknesses can be controlled precisely, reflective films having desired reflective and transmissive characteristics could be produced.
Several of the patents and applications discussed above contain teachings with respect to introducing layer thickness gradients into multilayer polymeric bodies. For example, Alfrey et al describe multilayer reflective films having layer thickness gradients. The methods suggested for creating the gradients include embossing of the film, selective cooling of the film during final stretching, and using a rotating die to create the layers (with reference to U.S. Pat. Nos. 3,195,865; 3,182,965; and 3,051,452). The Schrenk et al '985 and '707 patents also mention embossing and differential cooling.
However, the techniques of embossing and selective cooling relate to attempts to introduce layer thickness gradients into an already extruded film. Such techniques do not permit precise generation or control of the gradients. The use of a rotating die produces layer thickness gradients as a natural consequence of using the rotating die, with thinner layers being located nearest the rotating die boundaries. Again, however, the gradients produced cannot be precisely controlled.
Other methods have been used in the past which have introduced layer thickness gradients into a multilayer polymeric article. For example, Schrenk et al, U.S. Pat. No. 3,687,589, teaches the use to rotating or reciprocating shear producing means to vary the volume of material entering the feed slots in a coextrusion feedblock where the polymer streams are subdivided. However, precise control of volumetric flow rates using such devices is difficult to achieve. More recently, Schrenk et al, U.S. Pat. No. 5,094,788, teach using variable vanes in an interfacial surface generator downstream from a coextrusion die to introduce a layer thickness gradient into a multilayer polymer melt stream.
However, needs still exist in the art for methods and apparatus for generating precisely controlled layer thickness gradients in multilayered polymeric bodies which are useful as optically reflective articles.